Many strongly coupled fluids are known to share similar hydrodynamic transport properties. In this work we argue that this similarity could extend beyond hydrodynamics to transient dynamics through the presence of non-hydrodynamic modes. We review non-hydrodynamic modes in kinetic theory and gauge/gravity duality and discuss their signatures in trapped Fermi gases close to unitarity. Reanalyzing previously published experimental data, we find hints of non-hydrodynamic modes in cold Fermi gases in two and three dimensions.In the past decade, precision experiments of ultracold quantum gases at unitarity have enabled the study of transport phenomena in strongly coupled systems [1][2][3][4][5][6]. Usually, transport phenomena are encoded in hydrodynamic transport coefficients, such as the speed of sound, shear and bulk viscosities, heat conductivities, spin diffusion coefficients, etc. Presently, the speed of sound has been measured in a three dimensional unitary Fermi gas [2,6], and the shear viscosity has been constrained for both three dimensional and two dimensional Fermi gases [4,5]. The extremely low values of shear viscosity (when expressed in units of entropy) in particular suggest similarities between cold Fermi gases close to unitarity and very different systems such as hot quark gluon plasmas [7,8], high-temperature superconductors [9] and strongly coupled fluids described by black holes via the AdS/CFT conjecture [10], all of which share similar transport behavior. This apparent similarity in otherwise completely different physical systems suggests that these systems could be part of a broader class of so-called strongly interacting quantum fluids (SIQFs).It is conceivable that SIQFs share other properties besides their similar (hydrodynamic) transport behavior. This could be important because it could imply that it is possible to learn about one example of SIQFs (say high-temperature superconductors) through studying a different SIQF for which a particular trait is more easily accessible. In the present study we will investigate ultracold Fermi gases close to unitarity and argue that they exhibit properties similar to black hole SIQFs.One property that is quite remarkable about black hole SIQFs is that they do not seem to possess a description in terms of weakly coupled quasiparticles [11]. Instead, black holes can be characterized in terms of their ringdown spectrum, similar to a glass struck (lightly) with a fork [12]. Some of these quasinormal modes can be recognized to be the well-known hydrodynamic modes, i.e. sound and shear excitations. Others do not have an equivalent in (Navier-Stokes) hydrodynamics, and are thus non-hydrodynamic, but nevertheless affect transport properties (particularly on short time scales).If properties of SIQFs are universal, one would expect the presence of non-hydrodynamic modes in cold Fermi gases close to unitarity. This provides the motivation for searching for non-hydrodynamic modes in cold Fermi gases, both theoretically and experimentally.Transport in hydrodynamics...

The search for a possible critical point in the QCD phase diagram is ongoing in heavy ion collision experiments at RHIC which scan the phase diagram by scanning the beam energy; a coming upgrade will increase the luminosity and extend the rapidity acceptance of the STAR detector. In fireballs produced in RHIC collisions, the baryon density depends on rapidity. By employing Ising universality together with a phenomenologically motivated freezeout prescription, we show that the resulting rapidity dependence of cumulant observables sensitive to critical fluctuations is distinctive. The dependence of the kurtosis (of the event-by-event distribution of the number of protons) on rapidity near mid-rapidity will change qualitatively if a critical point is passed in the scan. Hence, measuring the rapidity dependence of cumulant observables can enhance the prospect of discovering a critical point, in particular if it lies between two energies in the beam energy scan.A central goal of heavy ion collision experiments is to map the QCD phase diagram as a function of temperature T and baryon chemical potential µ B [1-3]. At zero µ B , the phase diagram features a continuous crossover from quark-gluon plasma (QGP) to ordinary hadronic matter as a function of decreasing T [4-8]. Increasing µ B corresponds to doping the QGP with an excess of quarks over antiquarks, and it is an open question whether the crossover becomes a sharp first order phase transition beyond some critical point [3,9]. At nonzero µ B where lattice calculations become extremely difficult [10,11], there are no first-principles theoretical calculations which provide reliable guidance as to the whether there is a critical point in the phase diagram of QCD, or its location if it does exist [12][13][14][15]. Model calculations suggest the existence of a critical point, but disagree wildly on its location in the (µ B , T ) plane [14,15]. Reducing the beam energy increases the µ B of the QGP produced in a heavy ion collision [3, 16-18] (principally because lower energy collisions make less entropy but also because they deposit more of their baryon number in the plasma) but it also reduces the temperatures achieved. So, these experiments can scan the crossover (and potentially critical) regime of the phase diagram out to some value of µ B corresponding to the lowest energy collisions that reach the crossover (critical) temperature [1, 2]. If a critical point is located in the regime that is within reach, it may be detected experimentally. The search for a critical point in the phase diagram of QCD at the Relativistic Heavy Ion Collider (RHIC) is currently underway, with collisions at energies ranging from √ s = 200 AGeV down to √ s = 7.7 AGeV, producing fireballs that freeze out with chemical potentials in the range 25 MeV < ∼ µ B < ∼ 400 MeV [17,18]. Phase I of the RHIC beam energy scan (BES-I) was completed in 2014, with no signs of a critical point for µ B < 200 MeV and with tantalizing but inconclusive results at larger µ B , in collisions with 19.6 AGeV ≥ √ s ≥ ...

We describe dipolar nematic colloids comprising mutually bound solid microspheres, three-dimensional skyrmions, and point defects in a molecular alignment field of chiral nematic liquid crystals. Nonlinear optical imaging and numerical modeling based on minimization of Landau-de Gennes free energy reveal that the particle-induced skyrmions resemble torons and hopfions, while matching surface boundary conditions at the interfaces of liquid crystal and colloidal spheres. Laser tweezers and videomicroscopy reveal that the skyrmion-colloidal hybrids exhibit purely repulsive elastic pair interactions in the case of parallel dipoles and an unexpected reversal of interaction forces from repulsive to attractive as the center-to-center distance decreases for antiparallel dipoles. The ensuing elastic self-assembly gives rise to colloidal chains of antiparallel dipoles with particles entangled by skyrmions.

No abstract